1. Field of the Invention
The present invention relates to a wafer spin coating system and, more particularly, to a spin coating system in which photoresist pumping operations are continually checked, and the spin coating operation is stopped when abnormal pumping operations are detected to thereby prevent the production of inferior quality wafers.
2. Discussion of the Related Art
It is known that when fabricating a semiconductor device, a wafer must repeatedly pass through various chemical and mechanical processing steps, e.g., oxidation, diffusion, ion implantation, photolithography, metallization and test processes.
During the semiconductor fabrication process, a photoresist coating step is performed prior to forming a mask pattern on the wafer. The photoresist includes a photosensitive resin, the main component of which is usually polyvinyl-silicon for semiconductor fabrication.
After a thin film of the photoresist is coated on the wafer, usually by spin coating, the wafer is exposed to light and developed. Either the exposed portion of photoresist is removed (in the case of positive resist), or the unexposed portion is removed (in the case of negative resist), to thereby form a desired photoresist pattern. Thereafter, photolithography or ion implantation is done using the mask pattern as shaped through the above process.
The particular thickness of the thin photoresist film coated on the wafer by the spin coating method is dependent on, for example, the viscosity coefficient and polymeric content of the photoresist, and the spinning speed and acceleration of the spin coater. When the spinning operation is complete, 80%-90% of the solvent contained in the photoresist is evaporated so that the thin film coated on the wafer is nearly in a dried state. After the thin film is completely dried, the wafer is subjected to additional processing steps.
A conventional system for spin coating the photoresist on the wafer is illustrated in FIG. 1. The conventional spin coating system can be generally divided into two parts, namely, a pumping unit 10 for pumping the photoresist from a tank or container and a spin coating unit 12 that receives and spin coats the photoresist on the wafer.
The pumping unit 10 is provided with an internal container 14 that is connected to a plurality of containers 16, 18 outside the pumping unit that are filled with the photoresist. The photoresist is fed into the internal container 14 of the pumping unit 10 from the external containers 16, 18 through valves 20, 22, respectively. This photoresist feeding operation is performed by reducing the pressure in the internal container 14 by discharging the air contained therein through a discharge valve 24.
The photoresist contained in the internal container 14 is fed to the spin coating unit 12 through a valve 26. The pumping force necessary for transferring the photoresist from the internal container 14 to the spin coating unit 12 is generated by pressurizing the internal container 14 with nitrogen gas (N.sub.2). The nitrogen gas is introduced into the internal container 14 through an open valve 28, while the other valves 20, 22 and 24 are closed. The increase in pressure due to the nitrogen causes the photoresist to be discharged to the spin coating unit 12 through the valve 26 and thereafter sprayed onto the wafer in the spin coating unit 12.
A controller 30 controls the opening and closing of the valves 20, 22, 24, 26 and 28 to feed the photoresist and the nitrogen gas to the internal container 14 in the pumping unit 10.
In the spin coating unit 12, the spin coating operation is performed by mounting a wafer on a rotation driving device 34 and rotating the rotation drive device 34 while the photoresist is sprayed onto the rotating wafer.
In operation, the controller 32 in the spin coating unit 12 receives a spin coating commencement signal, starts rotating the wafer, then outputs a pumping operation order to the controller 30 of the pumping unit 10. The controller 30 then closes valves 20, 22 and 24 while opening valves 26 and 28. The nitrogen gas is introduced into the internal container 14 to increase the pressure on the photoresist contained therein. The pressurized photoresist is then fed to the spin coating unit 12 through valve 26 and sprayed onto the wafer.
However, in the conventional spin coating systems, the wafer coating operation is performed and the pumping operation orders are sent to the pumping unit 10 without checking whether or not the photoresist is being sprayed on the wafer. As a result, even when the photoresist pumping operation is not performing normally, due to a system error for example, and the photoresist is not being sprayed onto the wafer, the controller 32 for the spin coater keeps rotating the wafer. This causes the condition of the wafer to deteriorate. Also, when the deteriorated wafer passes through the wafer coating operation without the photoresist being sprayed thereon, subsequent processing steps are negatively affected. As such, a large number of substandard wafers may be produced before the condition is identified and corrected.